The present invention relates to a time-division telephone exchange and more particularly to an immediate ring-back control system for a time-division telephone exchange, which allows a calling subscriber to receive a ring-back tone having no phase delay selected out of plural ring-back tones having various phases at the time of connection of ring-back tone to the calling subscriber so that the calling subscriber may receive the ring-back tone immediately after the calling subscriber has dialled a called telephone number.
It is desirable to shorten the period of time during which a calling subscriber has dialled a telephone number, a speech path has been established between the calling subscriber and the called subscriber, and the ring-back tone is received by the calling subscriber, that is, to reduce what is known as post dialling delay. The post dialling delay time includes, in addition to a period of time for establishing the speech path, the delay time from the beginning of the ready state of calling the called subscriber in a terminating office to the time of the transmission of the ring-back tone to the calling subscriber. This delay time has a significant influence on telephone service. For example, if the ring-back tone is sent intermittently with one-second mark and two-seconds space, a maximum of two-seconds delay time may be required before the ring-back tone is sent to the calling subscriber.
In order to reduce such delay time to improve the serviceability, a first ringing control system has been heretofore used in which a continuous ringing signal is sent to the called subscriber once for a predetermined period of time after the establishment of the speech path and the continuous ring-back tone is sent to the calling subscriber, and thereafter a periodic intermittent ringing signal and ring-back tone are sent. However, since the prior art ring-back tone sending system requires a first ringing control circuit for each of the ring-back tone trunk circuits and ringing trunk circuits provided therein, it has a drawback in that the cost of the ring-back tone and ringing trunk circuit is high.